The present invention relates to a golf ball mold and a golf ball manufacturing method which may be suitably used for molding golf balls composed of a core encased by a cover of one, two or more layers, particularly for forming an outermost cover layer having a plurality of dimples formed on the surface thereof.
In recent years, there has been a strong desire for various performance attributes in golf balls, including not only distance, but also controllability, durability and feel on impact. Satisfying all of these attributes with only one type of material is generally difficult. Hence, the customary practice is to provide the ball with a structure in which a solid core formed of rubber, resin or the like, or a wound core, is encased by a cover of one, two or more layers, each layer having a particular function. In other words, efforts have been made to achieve a performance which addresses the needs of the player, such as a desire primarily for distance or for controllability, by adjusting the number and thickness of the above layers, and also by adjusting the formulations, etc. of the materials making up the respective layers.
An injection-molding method is typically used to form the outermost layer of a golf ball having such a structure. Specifically, use is made of a method in which a core or a sphere (referred to below as a “center sphere”) composed of such a core encased by one or more intermediate layer (a layer other than the outermost cover layer) is placed in the cavity of a given mold, and a cover-forming material is injected between the center sphere and the inner wall of the cavity. In this case, when the outermost layer is formed, numerous dimples are formed at the same time by numerous dimple-forming protrusions which have been provided on the inner wall of the cavity.
Up until now, molds having the structure shown in FIG. 22 have commonly been used when producing golf balls by the above process. FIG. 22, which is a cross-sectional view showing an example of a golf ball mold according to the prior art, depicts the state prior to injection of the cover-forming material into the cavity.
In FIG. 22, a conventional mold 10 is equipped with a mold body 20 having an upper mold half 20a and a lower mold half 20b which split at a parting surface that defines a parting line PL at a golf ball equator and removably mate to form a cavity 3 having an inner wall with numerous dimple-forming protrusions thereon, and support pins 40 which can be extended and retracted within pin insertion holes 50 that communicate with the cavity 3. The support pins 40, each of which is provided on an end face thereof with a single dimple-forming protrusion, extend into the cavity to support a center sphere 31. When the support pins 40 are in the retracted state, the end faces thereof define a portion of the inner wall of the cavity 3. Moreover, although not shown here, each support pin 40 has a circular cross-section, and three pins each are provided in the upper mold half 20a and the lower mold half 20b, for a total of six pins, so as to be spaced at given intervals at positions having 120 degree rotational symmetry about poles Q of the cavity 3 as the center.
In the above mold 10, runners 60 and resin injection ports 70 having openings of given surface areas are formed along the parting surface of the mold body 20 in such a way as to inject, between the inner wall of the cavity 3 and the center sphere 31, a known cover-forming material from a known injection molding machine (not shown). Next, together with injection of the cover-forming material, the support pins 40 that were extended into the cavity 3 are retracted, after which cooling is carried out, thereby completing formation of the cover. The dimples at the positions of the support pins 40 are formed at this time by the dimple-forming protrusions that have been formed on the end faces of the support pins. Gases within the cavity 3 are discharged through gaps arranged at given intervals that have been provided between the support pins 40 and the pin insertion holes.
However, when the center sphere 31 is placed in the mold 10 and the upper and lower mold halves are closed, the support pins 40 are often subjected to excessive forces, causing them to deflect or shift. In such cases, the support pins 40 move away from the center of their respective pin insertion holes 50, causing the gap between each pin and its hole to become uneven. Hence, during molding, material entry is concentrated at places where the gap between the two is wide, as a result of which flash forms unevenly on the surface of the molded ball. When uneven flash forms on the ball's surface, uniformly trimming the overall ball is not easy, making a clean appearance difficult to obtain. Moreover, this may also lead to a loss of dimple uniformity and ultimately have an adverse effect on flight symmetry.
Moreover, because strong chafing arises at specific places owing to contact between the support pin 40 and the pin insertion hole 50, at the off-center position of the pin 40, rubbing debris tends to form. When molding cycles are repeated in such a state, the rubbing debris that has formed may enter into the cavity 3 and ultimately contaminate the molded ball. Moreover, due to chafing between the support pins 40 and the mold body 20, abrasion of the inner wall side of the cavity 3 occurs, which may give rise to appearance defects in the molded ball.
Hence, problems caused by shifting of the support pins and abrasion can have a major influence on the quality of the finished product and on the maintenance period and life of the mold, and are thus a concern that directly impacts the cost of the product.
In order to resolve such problems relating to appearance defects and the like, JP-A 2002-542067 (and the corresponding U.S. Pat. No. 6,129,881), U.S. Pat. No. 7,341,687, JP-A 08-300403 and JP-A 2005-143610 disclose molds wherein the venting of gases near the poles of the mold cavity is improved by providing gas-venting pins in the mold body and in large-diameter support pins. However, the gas-venting pins and the support pins in such molds must be separately fabricated, resulting in additional costs. Moreover, such molds do not resolve the above problems caused by abrasion.
In this way, various modifications have hitherto been made to golf ball molds in order to improve the golf ball moldability and the mold life, but a fundamental solution has yet to be found for the above problems. Accordingly, for the sake of further enhancing golf ball moldability and mold life, and thus achieving further improvement in quality and lower costs, there has existed a desire for a novel approach which is capable of resolving the above problems.